This invention relates to improvements in the performance and reliability of GaN- and ZnSe-based multilayer structured semiconductor laser devices and light-emitting diodes. This invention also relates to improvements in the high-power output and manufacturing cost of GaN-based laser devices and light-emitting diodes achieved by applying the invention to the substrate structure of the GaN and ZnSe-based laser devices and light-emitting diodes.
The inability to produce p-type doping in ZnSe led to an inability to form a p-n junction, so that neither ZnSe-based blue laser devices nor ZnSe-based blue light-emitting diodes could be manufactured, although ZnSe had been expected to be a promising material for those devices and diodes. After the doping of active nitrogen into ZnSe succeeded in producing p-type ZnSe, the commercialization of ZnSe-based mixed crystal semiconductor devices became nearer to realization. At the beginning of the development of the devices, their operating voltages were very high, ranging from 40 V to 50 V, very different from the operating voltages of normally used semiconductor devices (1 to 2 V). The reason for this is that the upper valence band edge of a ZnSe-based mixed-crystal semiconductor device is very low, in terms of electron energy, and results in a very high contact resistance to the p-type ZnSe, and the resultant voltage drop across this barrier contributes to the excessive operating voltage.
To solve this problem, devices using semi-metal contacts, such as HgSe, or ZnSe-ZnTe graded superlattices as quasi-ohmic contacts to p-type ZnSe have been developed. However, even with those techniques the operating voltages of ZnSe-based mixed-crystal semiconductor devices are still as high as 7 V to 10 V, which means that they have to be further improved before they are put to practical use. To clarify the cause of the generation of the 5 V to 8 V, considered to be excess voltage, and to realize a practical device, various attempts have been made.
It has been considered that there is no discontinuity in the conduction band between n-type GaAs, serving as the substrate for a ZnSe-based mixed-crystal semiconductor device, and an n-type ZnSe layer grown onto the n-type GaAs, and thus no barrier exists to electron conduction in the direction perpendicular to the interface.
For example, in J. Ren et al., J. Cryst. Growth, 138 (1994) p. 455 and L. Kassel et al., Appl. Phys. Lett., 56 (1990) p. 42, it has been concluded that a band offset in the conduction band between GaAs and ZnSe is zero. In A. D. Katnani et al., Phys. Rev. B, Vol. 28, No. 4 (1988) p. 1944 and S. P. Kowakzyk et al., J. Vac. Sci. Technol., 21 (1982) p. 482, it has been concluded that the band offset is 0.2 eV.
Specifically, it has been considered that as shown in FIG. 1, at the junction between n-type GaAs and n-type ZnSe, the band offset exists mainly in the valence band and only an almost negligible discontinuity of about 0 to 0.2 eV is present in the conduction band.
The roles of buffer layers presently used in growing ZnSe on a GaAs substrate are only to reduce the density of point defects, caused in the grown ZnSe layer, and stacking faults leading to the device breakdown. Therefore, a method of performing the epitaxial growth of GaAs or InAs on a GaAs substrate as a buffer layer has been used.
The problem of the presence of an excessive ohmic voltage drop in ZnSe-based multilayer structured semiconductor devices contributes greatly to a decrease in the performance of current-injection laser devices and light-emitting diodes. Although GaN, like ZnSe, had been thought to be a promising material for blue laser devices and light-emitting diodes, a bulk semiconductor substrate presenting excellent ohmic characteristics at the interface with the grown layer has not been found, so that insulating sapphire, even with its large lattice mismatch, has been used for substrates for epitaxial growth. Since sapphire is insulating material, both of the electrical terminals are provided on the surface side of the device. This not only makes it difficult to achieve low-voltage and high-current operation, leading to a high-power operation, but also gives rise to a productivity problem and a high manufacturing cost problem.